NTC thermistors offer customised sensing solutions

Edited by the Engineeringtalk editorial team Dec 9, 2004

For the majority of applications, system designers may consider several temperature sensor types: this article compares the merits of NTC thermistors and metal element RTDs.

Temperature measurement and control is critical in diverse areas including industrial processes, environmental monitoring, medical applications, communications and computing equipment, automotive and aerospace applications, HVAC and refrigeration applications.

For the majority of applications listed above, system designers may consider several temperature sensor types.

Often a resistive sensor is the most useful, and for the temperature range of -50 to +250C, the sensors that are most likely to be considered are metal element resistance thermometer devices (RTDs), typically platinum sensors (Pt100s or Pt1000s) or NTC thermistors.

When comparing between NTC thermistors and metal element RTDs for sensitivity, a thermistor offers the best sensitivity of any temperature sensor type in the range from -50 to +250C.

This sensitivity is of the order of -4 to -6%/C.

In contrast, metal element RTDs have a sensitivity of approximately 0.4%/C.

In absolute terms, the sensitivity of an NTC thermistor can range from several ohms to several hundred ohms per degree Celsius, while the sensitivity of a metal element RTD is of the order of 0.4-4ohm/C.

Both an NTC thermistor and metal RTD can be made to a very small size.

In terms of mechanical and electrical performance, NTC thermistors remain durable and robust at small sizes, whereas metal RTDs are more fragile.

A large variety of NTC thermistor configurations are available.

This gives great flexibility in terms of electrical and mechanical parameters.

NTC thermistors are not subject to standards and a variety of thermistor materials are available.

Manufacturers of metal RTDs are constrained by standards, by choice of materials (typically platinum or nickel) and options for customisation are limited.

NTC thermistors can be implemented in a variety of sensing element sizes and lead wire types.

This allows for various levels of power handling.

Metal element RTDs typically have limited power handling ability.

NTC thermistors are generally more robust electrically and mechanically.

The combination of high basic resistance values and high sensitivity of NTC thermistors allows for easy interfacing to instrumentation without significant signal conditioning.

Metal element RTDs typically require signal conditioning and amplification and are more susceptible to electrical noise effects.

The operating temperature range for NTC thermistors is from -100 to + 300C.

Metal RTDs offer a wide temperature range from -260 to +850C.

Metal RTDs are usually more expensive than NTC thermistors.

An NTC thermistor can be configured with sizes and lead types that offer very fast response times.

NTC thermistors offer tight tolerances (typically +/-0.1C, +/-0.2C and customised values).

The large temperature coefficient and high basic resistance values of an NTC thermistor means that lead resistances are usually negligible in temperature sensing applications, even where the NTC thermistor sensor is used in probe configurations with lead lengths of up to several meters.

NTC thermistors can be used in a two-wire connection scheme without compromising accuracy.

Metal element RTDs typically require three- or four-wire connection schemes to achieve reasonable accuracy.

Both NTC thermistors and metal RTDs offer high stability.

NTC thermistors have the greatest potential for implementing customised sensing solutions that give a reasonable combination of accuracy, durability, interchangeability and ease of interfacing, over the temperature range -50 to +250C.